The present invention relates to a data communication system and, more particularly, to a data communication system applicable to a copier for implementing the interchange of data between a master CPU (Central Processing Unit) and a CPU which supervises optics.
Inexpensive and high performance CPUs are available in various forms today, thanks to the recent progress in the semiconductors art. Hence, a current trend is toward a distributed control system which controls a machine by using a plurality of CPUs in place of a single CPU type control system. When a pluality of CPUs are used in combination, it is necessary for them to interchange data with each other and this kind of data communication has heretofore been implemented by any of three different systems: a parallel data communication system, a serial data communication system, and an interrupt data communication system. A parallel data communication system is such that a plurality of signals, e.g., eight bits or one byte in the case of an 8-bit CPU are transmitted transmitted or received at the same time. Although this kind of communication system is advantageous over a serial communication system because it allows a large amount of data to be interchanged within a short period of time, it needs a plurality of ports and therefore has to withstand a substantial load with respect to hardware (buffers, connectors, harness, etc). The serial data communication has a drawback that sequentially feeding a pluality of signals over a single line results in a lower rate of data communication than with the parallel data communication and is therefore unsuitable for mass data communication. The interrupt data communication system is higher than the other systems mentioned above with respect to the data communication rate. However, this type of system is unfeasible for the interchange of a plurality of different kinds of data because a CPU usually has only one or two interrupt terminals. When the interrupt terminals are used, there is a fear that unexpected signals are accepted due to noise.